Posted
by
timothy
on Tuesday November 13, 2012 @01:30PM
from the when-google-images-isn't-enough dept.

First time accepted submitter jemenake writes "A friend of mine teaches electronic media (Photoshop, Premiere, etc.) at a local high-school. Right now, they're doing Photoshop, and each chapter in the book starts with an 'end result' file which shows what they're going to construct in that chapter, and then, given the basic graphical assets (background textures, photos, etc.), the students need to duplicate the same look in the final-result file. The problem, of course, is that some students just grab the final-result file and rename it and turn it in. Some are a little less brazen and they rename a few layers, maybe alter the colors on a few images, etc. So, it becomes time-consuming for her to open each file alongside the final-result file to see if it's 'too perfect.'" How to look for images closer than they should be to the original? Read on for more details.

jemenake continues: "When I first discovered that she was doing this, my first reaction was that there's got to be some automated way of catching the cheaters. Of course, my first idea of just doing MD5 hashes of each file won't work, since most kids alter the file a little bit.

A second idea I had was to alter the final-result file in a way that isn't obvious, like removing someone's shoelace, mis-spelling a word in the background, or removing/adding some dust-specks. (I know map publishers and music transcribers use this trick to catch copiers). But this still requires that she look for the alteration in each file. I'd think that Photoshop, after all these years, would have some kind of scripting language which also supports some digital watermarking, but I've just never dabbled in that realm.

And, of course, I guess another solution would be for her to not provide the end-result file in Photoshop format, but to export it as a flat image. But I'm still intrigued by the notion of being able to "fuzzily" compare two photoshop files or images to find the ones which are too similar in certain aspects (color histograms, where the edges are, level of noise, whatever).

Perceptual Image Diff [sourceforge.net] and Find Image Dupes [freecode.com] might be helpful. If she runs finddupes with a threshhold of.99 or so, then it is likely just trigger on nearly exact copies. At least, it should narrow down the ones she has to inspect in more detail. On the other hand, pdiff will detect exact or nearly exact copies by specifying how many pixels are allowed to differ (so it can be fooled by addition of random noise). While pdiff is available for Windows as well as Linux, it seems that finddupes is Linux only.

On the grading machine, keep the history window open. It's stored as part of the file. File history should give a very good idea if the student is resorting to shenanigans. Yes, a student could delete the file's history, but the teacher could require 'showing your work' through the history.

Let them use their own base images! And then let them do something creative with them!

One of the least interesting and least creative classes I took in art school was one that was about producing photorealistic oil paintings based on photographs. The class was 99% about mechanical technique, and to hell with creativity... which seems to be the theme of the class being taught here. So be it. But at least the instructor let us pick our own photographs to replicate! So we'd have an interest in what we were doing. And even if he had never checked on our progress along he way (like would happen in any worthwhile "learn how to ____" class), he would know whether we had done the work, because each of our paintings was a) unique, and b) matched the photograph we'd had approved at the start of the assignment. Plagiarism wasn't even a question, and not just because we were working in traditional physical media.

All of these suggestions for how to identify plagiarism through technological measures are missing the point. The problem isn't "how to catch a cheat", but "how to give students an assignment that they will have a reason to bother doing in the first place".

All of these suggestions for how to identify plagiarism through technological measures are missing the point. The problem isn't "how to catch a cheat", but "how to give students an assignment that they will have a reason to bother doing in the first place".

This is a great idea, unfortunately it falls flat in the face of reality. I used to teach computers, and spent a lot of time coming up with (what I thought was) neat assignments. Students would photoshop themselves into historical photos. They would creat

Newsflash: Students usually cheat because they DON'T have a strong grasp of the task.

Not necessarily. There are many reasons that people cheat:1) They are lazy2) They "don't have the time"3) They think they're getting away with something

In these cases they use "I don't know how to do it" as the excuse to just cheat, rather than expend the effort required to ask for clarification or practice further until they do grasp the task/concept that they are performing.

For some reason in college (at least my college), people cheating is totally normal and students talk about it like it's no big deal.

It's of course easy to invent your own exercises, but even better would be to have the students to use pictures they have taken themselves to be used in the exercise. And almost everyone has a mobile phone with a camera these days so that would be a minor problem. Or provide a collection of pictures that can be used in the exercise and let them play around.

Just state the basic points, then let each student do what they can and let them rate each others results. Don't force the students to use the same template, let them have their artistic freedom.

And isn't the whole point behind the exercise to learn how to use Photoshop and other tools - not to try to mimic a creation?

(I am a music teacher, but want to add to the discussion from an arts perspective)

Not every person in every discipline should re-invent the wheel at all times. Though as an expert (assumably) you should be coming up with your own material, there are other instances where there are resources which are as good as anything you'd put together on your own. Including assignments.

In this case, however, I think there's an important consideration. Even if the primary focus is to learn Photoshop, this is an arts c

There are some good problems that have been asked over and over again because they teach good lessons. My data structures professor started one of our assignments off with the following quote "More time has been spent on undergraduates recreating the Ackerman function than any other problem in computer science, and you all will be no different"

Sure there are other problems that have double recursion but why try to find something new and different when a good problem already exists? Plus there is something unifying about it. If I meet someone who graduated years before me or years after and they also had to do the Ackerman function in some language maybe the same one I used it kind of give you something in common. I like that; a common thread the ties us all together.

I believe that what you're complaining about, is exactly what separates excellent teachers from mediocrities who are looking for tenure. A teacher who is incapable of making up his/her own exercises probably doesn't understand the content that they are teaching well enough to be teaching!

To be fair, it may take a lot of time to create an exercise. So - a good, motivated teacher does two or three such exercises this year, and saves them. Next year, two or three more - and saves those for future use as wel

I believe that what you're complaining about, is exactly what separates excellent teachers from mediocrities who are looking for tenure. A teacher who is incapable of making up his/her own exercises probably doesn't understand the content that they are teaching well enough to be teaching!

To be fair, it may take a lot of time to create an exercise. So - a good, motivated teacher does two or three such exercises this year, and saves them. Next year, two or three more - and saves those for future use as well. In five years, Teacher has all the textbook examples, plus a library of his own to draw on. Teach isn't restricted to the material that the students find in their textbooks, he has tools at his disposal to help him find the cheats.

The textbooks and workbooks are only aids for a good teacher, not the crutches that most mediocre teachers rely on.

Nice theory. If it's true, so what? The reality of the situation is that we also rely on mediocre teachers, because there simply aren't enough excellent ones to go round.

But is it true? Well, as a teacher, I'm predisposed to say "no", amn't I? So it'll be no surprise that I say "no", then. First up, you haven't even addressed the core problem here -- even if the teacher makes his own tasks, he's still reusing them, and they're still going to be the same for all his students.

Sure. And it's hard to build a well constructed chest while being really easy to build a shoddy one. Yet we still expect people who we pay to build chests to build them well. I'm not sure why that doesn't apply to people whose job is to teach being expected to perform the various parts of teaching well.

But it seems like a trivial situtation to me - then again I'm don't do photoshop or art so maybe there's something tricky. Just give the students the flattened compressed to crap jpeg export of the final imag

Or require students to also hand in the intermediate steps for the homework just like old school math.

At the start of the course have a discussion about ethics and expectations. Have a class discussion of the purpose of the exercises. Have the class participate in designing the evaluation scheme (percentages for HW, tests, etc.) Get them to buy into the course so they view it as something they are participating in because they see value in their participation. Have them turn in some intermediate steps, and maybe some commentary on things they found challenging or interesting about the activities.

Record transgressors and use the policies of your institution to at the very least get it into their institutional record if they commit any accredited dishonesty so that if they have a pattern of that type of behaviour they can be tracked.

Record transgressors and use the policies of your institution to at the very least get it into their institutional record if they commit any accredited dishonesty so that if they have a pattern of that type of behaviour they can be tracked.

I meant to say "academic dishonesty" rather than "accredited dishonesty", though if you can get their dishonesty "accredited", more power to you.

Give them the final image as a flat raster image file, say JPEG or PNG. Since they must be turning their work in as a PSD (Photoshop format) or similar file if it has layers and all that, no problem at all. If for some reason you need to include a sample PSD file so they can see what different layers do, make it of a completely different image.

Don't forget, you may have someone in the class who really is willing (sometimes) to do 3 times as much work

Upvotes if I had them - this is exactly what the difference blend mode does. You could even record the flatten/paste original/adjust blend mode/save as jpeg operation as an action run it on the folder of student images using the batch processor to produce a nice little set of comparison images all at once.

Also, doesn't the extended edition have some advanced quantitative analysis tools? I'm not sure what exactly is their scope, but when it comes to calculating differences between images, this sounds like it could be of some help.

Bingo. Pixel differencing will show which pixels...are different...which will show gradient differences (as broad areas of different pixels), layer positioning differences (as lines), etc.
Otherwise, I think it's pretty obvious that one does not provide the students with a final.psd with all the layers intact. At the least, any provided file should be a flattened version (PNG or JPG of decent quality) with a watermark...
There's just so many ways to thwart this. Does the PS instructor *know* image manipu

Depending on what the exercises are, they might not leave much room for slight differences. "Construct a rounded rect button with 12 px radius corners with a vertical gradient from #RRGGBB to #RRGGBB and a 15% drop-shadow with radius of 7px offset by 14px at 120."

A better solution is to digitally watermark the solution files, or if the students have access to the solution files independently (i.e. they came with the book on a CD), watermark the source files and require students to start with those rather t

How about simply not giving them the final file? Why not a printed copy?If they must have an electronic version of the picture give them a low res thumbnail version.Project the image on a screen and tell them to draw that.

Your problem is that you are over thinking the tech angle when low tech methods will be super effective.

The stated problem was how to stop people from copying the example file and handing it in as their own work. The suggested solution solves that problem perfectly. (unless you think it's easy to de-watermark a png and then separate it out in to believable layers in a psd file? (or at least easier than just doing the assignment))

How to stop students from cheating by copying each other is a completely different problem, and luckily, not what the submitter asked. (because there really isn't a "good" way in that

If you want to spend more time on it, make sure that the student's copy of Photoshop is set to record history in the metadata.

Then you can go through and look at every step they made. I do this on some images so I can figure out what the hell I did to get that effect three years later. It takes up little space - it's just text.

Provide the book resources as a tutorial but get the students to do something different for the actual assignment. It could be as simple as swapping a few textures or effects. A blur on a cat will look very different to a blur on a dog even though the technique is the same.

findimagedupes [jhnc.org] compares a list of files for visual similarity.

To calculate an image fingerprint:

1) Read image.
2) Resample to 160x160 to standardize size.
3) Grayscale by reducing saturation.
4) Blur a lot to get rid of noise.
5) Normalize to spread out intensity as much as possible.
6) Equalize to make image as contrasty as possible.
7) Resample again down to 16x16.
8) Reduce to 1bpp.
9) The fingerprint is this raw image data.

To compare two images for similarity:

1) Take fingerprint pairs and xor them.
2) Compute the percentage of 1 bits in the result.
3) If percentage exceeds threshold, declare files to be similar.

Of course, you shouldn't take its suggestions at face value every time, but it should help narrow your search for cheats.

"If they can't do it at the end, they will fail. If they didn't ask for help, tough. Treat them like adults. They will ether rise to it or learn what doesn't work."

While that is a fair approach, I've found that I get much higher success rates in the end if I have some kind of feedback during the period they are working. If they don't *know* they're way off track, or don't *know* they aren't putting in enough effort, then, sure, they'll fail at the end like they should, but there will be some missed opportu

In creative pursuits like Photoshop, you have subjective quality concerns. If you copy and paste a head from one picture onto a body in another and there's huge jaggies around the head, and no color matching, you might still be dumb enough to not realize you're off track. Or, get the edges perfect and still don't realize that it's not color matched, or that the shadow directions don't match.

As a teacher, I'd like to say that this method really, really worked for me at a major midwestern university. Then I moved to the South and tried the same method; it did not work. I'm not saying it's a regional difference, perhaps just admissions policies. But I'm at a second Southern university now, and I'm surprised students even wipe themselves, as they'll do little else if they don't receive a grade for it. Maybe this teacher works with similar students, ones for whom only high-stakes grading is suffici

I'd guess the students previous teachers have failed to teach them 'what doesn't work'. A teachers job is to teach students how to learn without a teacher. Any facts or skills they get along the way are just bonus's.

My sympathies. Perhaps high stakes chapter tests. I'd be damned if I would grade homework for fucking college students.

There are other algorithms that work on the final result to tell you how much of the photo was changed.

Since she has the originals, something like this would work: Auto compare every final picture with its original and produce a number, percent changed. Then use ComparePSD to compare the submission that are the same % changed.

The problem is that if she is too specific in her instructions for the assignment, then everyone in the class that tries will have the

I understand that the question as is left at the end of the submission is just a case of curiosity and there's plenty of good answers to the question here.

But the problem being described is entirely separate from that question - and the problem seems to be that there's a teacher who sets time-consuming tasks but does not want to do time-consuming review.

for one assignment students turn in their own pictures with themselves in it near some assigned object. Later, another assignment has them work with their picture toward some given result on the object, with them still in the picture.

for one assignment students turn in their own pictures with themselves in it near some assigned object. Later, another assignment has them work with their picture toward some given result on the object, with them still in the picture.

It's a good chance that the more photogenic students will get the highest grades. There are several studies about grad student grant proposals that back this up.

> So, it becomes time-consuming for her to open each file alongside the final-result file to see if it's 'too perfect.'"

How is it that she's grading these? One would assume the grade depends on similarity to the target image or the layers embedded in the file [1] which should be dependent on comparing the student file against the master.

Or is it yet another "Best try" scheme? "You tried, Timmy, so I give you an 'A'".

[1] Does Photoshop embed the history inside the file? It's been awhile since I've work

And, of course, I guess another solution would be for her to not provide the end-result file in Photoshop format, but to export it as a flat image. But I'm still intrigued by the notion of being able to "fuzzily" compare two photoshop files or images to find the ones which are too similar in certain aspects (color histograms, where the edges are, level of noise, whatever).

If you provide the kids with the end result, and they need to turn in an end result for grading, you're fighting a losing battle. I could personally get around everything you did to try to protect your "example" PSD, and I'm relatively certain that I could have done so at the age of your students. Just give them the flattened image, it's enough for them to see what it should look like.

You should still, of course, use one of the methods mentioned in sibling posts to compare submissions for too much similar

Do what I do for my textures, and embed a "watermark" of your signature or something similar deep into the final image where it can't / won't be seen by anybody who doesn't know where or what to look for, in multiple places where the pixels are conducive to such masquerading. It's almost a form of steganography, where the message to be sent is a verification of the authors' identity and claims of original work.

I do mine in such a way that even if I leave one such image that can be readily seen, there are at least a half dozen more than cannot be found without a side-by-side comparison of source and production images with and without the "watermarks" (impossible without someone getting hold of my.PSD's). Keep the true "source".psd for yourself, create another for disbursing to students that contains several "watermarks" with an extreme level of transparency well-blended into many or all of the layers so they'll have an example.psd to "reverse engineer", and then separately give them the actual un-watermarked original source images, which they should then be expected to chuse to assemble the final image themselves. You might even put an entirely separate watermark into the source images, so you can check to see which watermarks the submitted image has, as opposed to checking only for the source mark.

If they put in enough time and effort to actually successfully circumvent this technique by finding and either eliminating or duplicating all the various marks, then they've probably got the requisite skills to pass the original challenge... at least if you do it the way I do.

My "signature" is in at least 3 places in this image [photobucket.com], buried deep in different layers with heavy transparency masks, and it would have to be altered drastically to be guaranteed to remove all traces of it.

A few years back I used iPhoto and it had facial recognition software built-in. When I went through training it, it mis-labeled faced but it did so along family lineage. For example, it would think my dad was me or vice-versa.

I currently have a program called PhotoSweeper (http://overmacs.com/photosweeper/) which uses five different methods to find duplicate images. It doesn't use facial recognition but instead it compared the bitmaps and/or histograms with a user-changeable threshold (e.g. identify re

You can use a difference filter, which will produce a ratio of dark/light based on the amount of difference, and then a histogram to get a more quantitative view of how much of the image is different, rather than analyzing with your eyes. You could do this on both flattened composite comparison, as well as layer by layer(maybe trying all combinations of layers and picking out the X number that are closest, where X is the number of layers in the final image). I'm not sure what the capabilities of Photoshop

It's probably annoying for all involved, but just like the "show your work" in math classes, you can request a "show your work" equivalent via screen-cast. And the students will learn a bit about screen-casting.

A second idea I had was to alter the final-result file in a way that isn't obvious, like removing someone's shoelace, mis-spelling a word in the background, or removing/adding some dust-specks. (I know map publishers and music transcribers use this trick to catch copiers). But this still requires that she look for the alteration in each file.

Ummm, maybe I'm missing something here? She should be looking at the file, right? What's the harm in checking one little thing as she's looking at it? I mean, how else is she going to be sure they actually did it properly without checking it anyway? Zero solutions should allow her to skip checking the homework entirely, if it does, its kind of missing the point.

While the flat no-layered file is the obvious solution, it will have an unintended side effect of not acting as a fall-back guide for students w

I use this http://www.duplicate-finder.com/photo.html to find image files which are similar (they don't have be identical). Does not work with PSD files though. Maybe the files can be exported to PNG, etc?

For one thing, you can prevent plagiarism by not asking students for plagiarism. You're giving students a file and then asking them to duplicate it. That's pretty much the definition of plagiarism and, frankly, probably of very little educational benefit.

The teacher needs to stop trying to figure out ways to catch people cheating on an exercise designed for cheating and start teaching the damn course. Teaching doesn't just mean lecturing and assigning exercises out of some book, it means developing exercises, homework problems, and exams from scratch as well.

Ummm, I could have sworn that computers are supposed to be deterministic things, in that if you start with the same inputs (the images) and do the same things to them (the steps they're told to do in the book) you will always get the same results.

How can the product be "too perfect" when the products should be identical? Ok, date stamps and maybe embeded path names will be different, but the image itself, starting from the same sources, doing the same things, should be the same.

Why does this sound like a stock image supplier trying to find machine-modified infringing images using a web crawler so that they can bludgeon the people publishing the modified images, who have not paid a license fee, with a copyright infringement lawsuit?

I'm just saying, a good answer to the OP's question is going to mean the ability to use the answer in this fashion.

Honestly, the best way to catch the cheaters is to test them. If they aren't doing the work, they aren't learning the steps. So hold a few tests throughout the term and make the tests worth more than the assignments. Show them, not give them but show them on an overhead projector or using a large photo, the end result that is required. They have 30 minutes to produce it.

Rather than having them copy the output. Give them each a different set of art assets, and have them each turn out an original work using the lessons -taught and shown- by the example. Then there's no copying, and they might actually learn something.

This is basically the digital equivalent of "show your work" on a math test. If you want to see how well the students are grasping certain concepts, tell them to include an audio track in which they describe what they're doing and *why* -- e.g. "lightening this layer now because I did *blah blah* and messed it up previously".

Anyone who is good enough to fake the screencast convincingly probably doesn't need this class. And if you're really concerned about people using a ringer to do their work, have an in-p

Duh. Flatten the final picture. You will still have outliers who can make it look convincingly different enough but it won't be as easy as giving them access to each individual layer for them to fudge up.

Also consider embedding tiny watermarks in the image. Little sets of pixels at specific coordinates. Not specifically exact colors but something you would be able to identify on close inspection. Something like a small 6x6 checkerboard grid of alternating light colored pixels in a light area of the document that wouldn't be easily seen by your students but you could zero in on and verify.

I think you should tell them at the outset "It's really easy for you to cheat on your assignments. That's also a horrible way to learn. I've got an honor system, don't cheat. If you do cheat you will learn less, and therefore be wasting your own time."
If you need something to base grades on, you need something else that you can watch then do or they can't cheat on somehow.

I think the problem is that while the assignment might be new, everyone in the same class gets the same original file, and has to make changes to the same specifications. I'd be supprise if some students didn't legitamately have the same end result.

One poster suggested that each student gets a slightly different original file. Something like different house numbers, but in the same font and color. If two students submit the same house number, you know they copied.

Also make it clear early-on that this class is designed to teach them a useful, marketable skill and that if they cheat, they won't have learned the skill and if enough of them cheat and don't get caught, YOU won't know to slow down the pace of instruction. As a result, the whole class may "pass" knowing a lot less than they would if nobody cheated.

Some artists/journalists would argue that *any* use of photoshop is cheating. Albeit, it's not copying, but you are bending reality. (Not that photo manipulation is new to photoshop. Trick photography started just a few years after photograpy itself was invented, and even realist painters didn't paint the "real", reality)

I agree - a blank slate is the best, in the real world you have to be creative.

Just state that they should have a picture with an animal, a beverage and a well-known landmark and that some types of transitions and effects are expected. But then also state that they aren't limited to that but can do something completely different as long as they have a certain number of effects in the image.

And to make sure that they don't copy an existing image they should provide the source images used too.

Exactly. The OP illustrates multiple failings with the teacher (exercises that are nothing more than copying, attempting to avoid actually looking at images, etc.) and then attributes the problem to the students while jumping over the fact that the students probably aren't learning a god damned thing in class.